Carrying Up a Heavy Load
It’s interesting to see how the crews at Vaughn Construction, Elkus Manfredi Architects and Kirksey Architecture are able to help The University of Texas M.D. Anderson Cancer Center (MD Anderson for short) create South Campus Research Building 5 (SCRB5) as I discussed in the previous article I wrote. Being able to construct such a building with all the geotechnical evaluations, the structural members of the building, and other necessities such as the building’s HVAC system, plumbing, fire sprinkler systems, electrical and telecommunications infrastructure is truly a monumental task. It’s easy to see how the base of the research building is created, but how are they able to carry up such heavy equipment to the second and third floors? These companies and organizations don’t have the means to hire 100,000 men part-time for twenty years like the Egyptians did when they were building the Giza Pyramids as the Greek historian Heroditus once claimed. How can construction crews build such massive structures in comparatively small time frames, even when taking into consideration that these modern-day structural behemoths contain more equipment in them than in the times of ancient Egypt? Moreover, how can they build such a structure in a densely-populated urban environment?
The answer: construction cranes. These machines are able to effectively and efficiently carry large items that weigh many times more than the machine itself without causing damage to the construction site, the machine, or hurt any workers on site. They are also suited to work in tight spaces due to their design which makes them ideal for urban environments. They come in a variety of different forms like, as Maxim Crane Works clearly illustrates and describes, static cranes consisting of Hammerhead Tower Cranes, Luffing Jib Cranes and Self-Erecting Cranes; the mobile cranes, on the other hand, consist of cranes like the Crawler Cranes, the Rough Terrain Cranes and the All-Terrain Cranes.
However, in this article, I want to focus on the Hammerhead Cranes, the type of Tower Crane that Vaughn Construction is borrowing from Morrow Equipment Company. How do these machines work exactly? What engineering principles are allowing these machines to effectively stay up in the air without falling over and causing damage to those below who would be subject to its wrath? Is it a balancing act? Does the way their structure is built allow them to be so sturdy to where we don’t have to worry about anything failing?
The answer to these questions lies somewhere in the middle of all these questions. Let’s dive in and use the engineering principles from Statics such as the Method of Joints and the Method of Sections to understand the Hammerhead Tower Crane’s Truss Structure, Tension in the wires that help stabilize the jib and counter jib (see illustration below), Moments-not Moment of Inertia-to better understand the balancing act that is needed for the crane to operate effectively, some material science to help determine what materials work best for a Hammerhead Crane to effectively withstand the various heavy loads and forces, a little bit of fluid dynamics in understanding how pressurized fluids can be used to lift and rotate the crane, and also analyzing how the Hammerhead Crane was engineered to be mobile and versatile so that it can be used in various environments.
Image courtesy of Mohamed El Mehdi R.’s “How Tower Cranes are Erected - A Step - By - Step Guide” on LinkedIn
Using Tension and Compression to Hold It Together
Cranes are unique in that they employ a type of structure known as a truss. As is the namesake of this website, trusses are one of the most, if not the most, efficient engineering structures in the world. They are able to carry so much weight relative to their size. How are they able to do this? Trusses are designed as either a square with a diagonal member in the middle or as interconnected triangles. The diagonal members of the structure are really what give the truss its rigidity. It’s so rigid and durable that many bridges around the world incorporate this structure, including the Ikitsuki Bridge in Japan and the Astoria-Megler Bridge at the Oregon-Washington border in the United States.
Ikitsuki Bridge, Japan (Courtesy of Engineering News-Record and Wikimedia Commons)
Astoria-Megler Bridge, Oregon-Washington, USA (Courtesy of Engineering News-Record and Wikimedia Commons)
For context, these bridges are known as continuous truss bridges, which tend to be more complex than a simple truss bridge due to the indeterminate forces within the bridge, or forces that statics alone cannot measure in a given structure. For now, we’re just going to stick with a simple truss to illustrate in a simplistic manner how these structures operate.
Each structural part of a truss is known as a member. Each member is grouped with two or three other members to help determine the forces exerted on each member. When forces on each member are calculated based on what force is exerted on each member within a particular grouping of members, the method is known as the Method of Sections. However, forces in members of a truss can also be calculated at a specific joint, either a roller or a hinge, where two or three members connect. This is called the Method of Joints. Here are two YouTube videos from Dr. Jeff Hanson with the Department of Mechanical Engineering at Texas Tech University that help explain these concepts further:
Statics: Lesson 48 - Trusses, Method of Joints
Statics: Lesson 49 - Trusses, The Method of Sections
As you could imagine, if the materials that comprise the truss structure are light enough yet are sturdy enough to support a high amount of load, the truss structure can be very effective, which is crucial for a crane that’s carrying and unloading heavy materials onto and away from a construction site such as the new SCRB5 high rise MD Anderson is building in the Texas Medical Center.
Speaking of tension, there’s also a cable at the top of the Hammerhead Crane that holds the Jib and the Counter Jib together. Similar to how some bridges have cables at the top to help keep the structure together, so does the Hammerhead Crane. Without it, the structure would be less stable overall, which we’ll get to in the next section.
An Elevated Balancing Act
As you can see from the diagram at the end of the introduction, the Hammerhead Truss jib and counterjib rest on the tower at their collective center of gravity. In order to ensure that the center of gravity never changes, a team of engineers will need to determine how much counterweight will be needed to offset the projected loads the crane will be subjected to. This is measured by determining the Moment-again, not the Moment of Inertia-at a particular point caused by a force along the combined length of the jib and/or counterjib of the crane.
If this sounds like torque to you, then you’d be on the right track: both are calculations of a force at a distance x or r away from the center and use the same units. Here’s the main difference though: torques are dynamic and move (think of a drive axle on a car, truck or bus) and moments are static and do not move. The point of calculating moments is to ensure that the structure is stationary and does not give to external forces. Take a look at this chart from BYJU’s which helps explain this concept further.
Before this structure is erected, however, a concrete foundation will need to be poured for the crane. This also aids in the Hammerhead Crane’s stability and resilience to external environmental factors. Once the concrete has dried, the base of the Hammerhead Crane will be built upon it. Here is an article from Andun Engineering Consultants that explains this process in much more detail.
So, what could happen if the Hammerhead Crane becomes unstable? Well, nothing good usually comes of it and there’s great potential for property damage and loss of life. This unfortunately happened in Dallas when a crane fell on top of an apartment building that resulted in the death of one resident and other residents being severely injured. Another example of a major crane failure comes from Ft. Lauderdale, FL where a section of a crane fell onto a roadway injuring several people and resulting in the death of one construction worker. When something as large as a Hammerhead Crane comes down, there’s bound to be damage and destruction.
Made With the Good Stuff
Because the crane is going to be subject to heavy loads and also tension and compression from within the structure, the truss is going to have to be made from materials that can handle these various forces. Furthermore, these materials need to be light enough to allow the crane to balance itself in the sky hundreds of feet above the ground and also allow the crane to move the jib and counter jib up with as little force as possible. The answer: high-strength low-alloy (HSLA) steels. According to Landwher Construction, Inc., these steels allow the members in the truss section to be welded together and, because these steels contain a lower amount of carbon, they are more durable and can last longer than other similar materials.
It’s also important to note here that, since welds create a weak point in the structure, using the right bead for the weld is of utmost importance when creating joints connecting two or more members in the structure. Proper selection of the weld’s bead will greatly reduce the chance of the truss’ structure to fail. Here are some examples from The Engineering Choice that explain the different types of welds and the different methods that are used to create these different welds.
Examples of weld beads (Courtesy of YesWelder)
While the article mainly discusses the different techniques a welder might use to accomplish a different weld, a crane manufacturer such as Liebherr might use specialized robots to weld the different members in the truss structure of the crane to maximize time efficiency. Nevertheless, the idea of using the correct weld bead still rings true in this scenario as well.
To Infinity…and Beyond!
It’s really neat how the crane can lift heavy objects with its truss construction, but how does the crane lift itself higher? How can the crane rotate and drop off a load at another location within the construction site? How can these massive machines raise and lower the different loads they are subject to? These towering machines use hydraulics to both lift themselves, their respective loads and rotate the jib and counter jib. More specifically, hydraulic jacks are used along the tower crane’s mast or tower to help lift the crane and place a new section into the mast. This can also be done in reverse when it is time to take apart the crane so the project can move on to other phases of construction. Here’s a video from Art of Engineering which explains in more detail how this process works:
How Tower Cranes Build Themselves
So now that we’ve learned the basics on how cranes lift and lower themselves, let’s see how these cranes rotate and carry heavy loads. The operator inside the hammerhead tower crane cabin can rotate the jib and counter jib as well as raise/lower a load containing equipment and materials needed to help get the job done. Thanks to the hydraulic systems on board, these cranes can perform these tasks effortlessly and efficiently. This short article from Texas Fluid Power helps further explain how hydraulic systems are important in rotating the crane and how they allow the crane to lift/lower heavy loads.
The concept used in these hydraulic systems comes from fluid dynamics where liquids can be pressurized but not compressed. In this scenario, the pressurized liquid will act upon another object with an equal and opposite magnitude thanks to Newton’s Third Law of Motion. Think of it like a hydraulic jack used to raise a car. You might use a reasonable amount of force to press the lever, but every time you do so, the hydraulic jack exerts a force on the car that allows the car to be raised higher off the ground every time you press the lever. Here’s a video from Deconstructed that shows in greater detail how this process works:
Hammerhead Tower Crane with cabin for crane operator (Courtesy of Crane Warning Systems Atlanta)
What Must Come Up Must Come Down
Construction projects won’t last forever and neither will the need for the hammerhead tower crane to be on site. Thankfully, these cranes are designed to be assembled and disassembled on site. Each member and part of the crane is meticulously engineered not only for durability and stability during its operation on a construction site but also because this level of precision allows the crane to be assembled and disassembled in the least amount of time possible. After all, time is money when you’re trying to finish a construction project. As the video from Art of Engineering illustrates, first a concrete base is poured in a strategic location at the construction site before construction begins that will allow the crane to be used to its fullest potential. Once the concrete is dried, construction of the crane will begin.
These components unfortunately don’t magically arrive on site by themselves. Instead, they need to be transported to and from the construction site. Before any of this takes place, meticulous planning and attention to detail are required for the crane to perform at its best. Here are some articles from AP Crane Training and West Coast Training which describe the logistics of transporting these cranes and all that is required to construct and deconstruct these machines on site.
Building the Future one Crane at a Time
Hammerhead Tower Crane on Construction Site (Courtesy of kurso.eu)
Cranes such as the ones that Vaughn Construction is using from Morrow Equipment Company are very beneficial to high rise construction projects. Because of the way they are designed, engineered and constructed, these machines effectively and efficiently utilize concepts from statics and fluid dynamics. However, it’s important to remember that, even though these machines have proven themselves many times for decades, there is still potential for detrimental effects to human life and the surrounding environment the crane is operating in if any part of the structure is compromised.
Thankfully, there are programs such as AP Crane Training and West Coast Training which help those on construction sites better understand how to safely construct and operate these machines. Similarly, it is important for the engineers designing and engineering these machines to carefully consider the effects of the materials being used, the construction methods used to create the different parts of the crane assembly and the physics governing the design in order for the crane to operate properly and safely.
How would you design and engineer a crane? Would you use materials other than high-strength low-alloy (HSLA) steel? Would you weld or use a different joining method to join the metal members together? Let us know what you’d do in the comments below!
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